The dashes and pairs of dots normally occupy separate sides, but in this structure, since there are more than four dashes and pairs of dots, some of the dots that surround chlorine must be placed at the corners. UCLA's Organic Chemistry Tutorials notes that in a Lewis structure, each dot represents an unshared valence electron, while a dash represents a single bond, an electron shared by two atoms. Chlorine has seven valence electrons, and three of them are used to bond with fluorine atoms in this molecule. Usually, substances share electrons to fill their outer shells with eight electrons, which is most chemically stable. ClF3 is very unstable because its central chlorine atom does not have a full shell.